In the field of mobile communication, circuit switched networks have traditionally been used, as e.g. in the GSM system. GSM has been steadily updated, and also more recently developed mobile communication systems such as CDMA and WCDMA (in accordance with the third generation partnership project 3GPP) make use of circuit switching.
Also, communication systems have been developed and are in development that are based upon the Internet Protocol (IP), such as the so-called IP Multimedia System (IMS). IMS is also standardized under the third generation partnership project. IMS based networks are being used to provide a plurality of services to the user of a mobile terminal, such as simple messaging or presence services, but more increasingly also telephony services such as Voice-over-IP (VoIP). As an example, it is presently already possible to provide an IMS based telephony service using WLAN radio access technology.
In order to address specific problems of selecting a proper subsystem or domain, the inventors of the present invention have described a basic service routing decision entity and service routing decision method in the earlier patent application PCT/EP2005/004515, the contents of which are fully incorporated herein by reference.
In technical document S2-050995 of 3GPP TSG-SA WG2 meeting no. 46 in Athens, Greece, May 9-May 13, 2005, a proposal was made for introducing a so-called Network Domain Selection functionality (NeDS) as a means for selecting a desired subsystem in which to terminate a mobile terminating call directed towards a terminal that can be reached via an IMS subsystem and a CS subsystem. The described network domain selection entity is arranged for conducting a subsystem selection procedure that comprises making at least one subsystem selection decision associated with a subscriber terminating service involving the terminal, for selecting one of the described subsystems or domains for terminating the subscriber terminating service at the terminal.
FIG. 9 shows a logical representation of the network subsystem selection functionality explained in technical document S2-050995. Reference numeral 90 relates to a network domain selection entity, 91 to an IMS subsystem, 92 to a circuit switched subsystem and 93 to a mobile terminal that has the capability of communicating via the IMS subsystem 91 and the circuit switched subsystem 92. The basic operation of the selection entity 90 consists in for example receiving an IMS multimedia telephony terminating request 901 and then deciding to terminate the call via the IMS subsystem 91 (arrow 902) or via the circuit switched subsystem 92 (arrows 903, 904). Equally, the entity 90 could receive a telephony call to the circuit switched domain 911, and could decide to route this call through the circuit switched subsystem 92 (arrow 912), or via the IMS subsystem (arrows 913, 914).